For me, it is Spilanthes, "Eyeball Plant" and any members of the Mimulus, 'Monkey Flower" genus. I can get them to germinate fine, but then they just sit at the first true leaves stage and never grow. I've tried both the Spilanthes and various kinds of Monkey Flowers 3 times, with no results, so I think I'm giving up. (Unless someone can give me a sure-fire tip. -LOL)
It was only my first year this year too, and I had about 4 things out of 54 not germinate. I'm not giving up on anything yet though; there are too many variables: temperature, snow cover, moisture, etc. to figure out in just one year what positively won't work...
Delphiniums and Larkspur. I can't get the Delph to sprout to save my life and I can't seems to keep the Larkspur alive once I plant it out. After a few years, you'd think I'd give up...well, I have given up on the Delphs, but I'm sure if I can just get the Larkspur in the right place, it'll survive. Oh, and Alyssum...I see people posting about getting tons of it...I get a few plants, but that's it.
Merryma~ do you have a moist area with afternoon shade then filtered late pm light? I was advised to tranplant my tiny W/S seedlings into an area like that.. Last year was the first year I ever had success with growing Larkspur. Here's one that I also grew in a container under a pine.
That is a beautiful flower! Unfortunately, most of my garden gets morning shade and afternoon sun...which keeps the house cool, but isn't the best for shade plants. I did start a new garden last year in the back on the north side of my house, so it gets mostly shade...just blazing late (very late) afternoon sun. That's the one place I haven't tried Larkspur. Maybe that's the spot it was waiting for. ;)